1. Field
The presently disclosed subject matter relates to an optical deflector including meander-type piezoelectric actuators.
2. Description of the Related Art
Generally, in an optical scanner or the like, an optical deflector is constructed by a micro electro mechanical system (MEMS) device manufactured by using semiconductor manufacturing processes and micro machine technology.
A first prior art optical deflector as a MEMS device is constructed by a mirror, a fixed frame surrounding the mirror, and a pair of meander-type piezoelectric actuators coupled between the fixed frame and the mirror and serving as cantilevers for rocking the mirror with respect to a rocking axis of the mirror (see: JP2008-040240A).
In the above-described first prior art optical deflector, when the position and direction of the rocking axis of the mirror are deviated from those of an axis such as an X-axis on the plane of the fixed frame while the mirror is being rocked, an irradiated area cannot accurately be scanned with light reflected from the mirror. Therefore, accurate control voltages of saw-tooth waves or the like need to be applied to the meander-type piezoelectric actuators, so that the cantilevers of one of the meander-type piezoelectric actuators are aligned symmetrically with the cantilevers of the other. In more detail, if another axis perpendicular to the X-axis is defined by an Y-axis on the plane of the fixed frame and an axis normal to the plane of the fixed frame is defined by a Z-axis, a tip of one cantilever on the side of one of the meander-type piezoelectric actuators and a tip of its corresponding cantilever on the side of the other meander-type piezoelectric actuator should have the same positions on the Y-axis and the Z-axis and opposite positions on the X-axis with respect to their origin.
However, it is actually difficult to symmetrically align the cantilevers of one of the meander-type piezoelectric actuators with those of the other, due to the manufacturing fluctuations between the pair of meander-type piezoelectric actuators.
Also, since the meander-type piezoelectric actuators are very brittle, the meander-type piezoelectric actuators would easily be displaced by an external impact. Therefore, when unexpected relative displacements occur in the positions of the meander-type piezoelectric actuators with respect to the plane of the fixed frame by such an external impact, the meander-type piezoelectric actuators would interfere with the mirror, and would be twisted or deformed to be broken down.
Next, the natural frequencies of the first prior art optical deflector will be explained with reference to FIGS. 1A, 1B and 1C and FIG. 2.
In FIGS. 1A, 1B and 1C, a mirror 101 is surrounded by a fixed frame (not shown), and a pair of meander-type piezoelectric actuators 102a and 102b are coupled between the fixed frame and the mirror 101. The piezoelectric actuator 102a is constructed by serially-coupled piezoelectric cantilevers 102a-1, 102a-2, 102a-3 and 102a-4 in parallel with the Y-axis of the fixed frame, and the piezoelectric actuator 102b is constructed by serially-coupled piezoelectric cantilevers 102b-1, 102b-2, 102b-3 and 102b-4 in parallel with the Y-axis of the fixed frame.
In FIG. 1A, which shows a normal mode whose natural frequency is fn0 as shown in FIG. 2, the Y-axis positions and Z-axis positions of the piezoelectric cantilevers 102a-1, 102a-2, 102a-3 and 102a-4 are the same as those of the piezoelectric cantilevers 102b-1, 102b-2, 102b-3 and 102b-4, respectively. On the other hand, the X-axis positions of the piezoelectric cantilevers 102a-1, 102a-2, 102a-3 and 102a-4 are symmetrical with the X-axis positions of the piezoelectric cantilevers 102b-1, 102b-2, 102b-3 and 102b-4, respectively, with respect to the origin O. Thus, the mirror 101 is rocked around the X-axis. Note that the natural frequency fn0 is determined by a mechanically-vibrating system of the mirror 101 around the X-axis depending upon the piezoelectric actuators 102a and 102b. 
In FIG. 1B, which shows a first abnormal mode whose natural frequency is fn1 (>fn0) as shown in FIG. 2, the Z-axis position at a folded portion Fa34 between the piezoelectric cantilevers 102a-3 and 102a-4 is opposite to the Z-axis position at a folded portion Fb34 between the piezoelectric cantilevers 102b-3 and 102b-4 with respect to the origin O. Note that the folded portion Fa34 and Fb34 are the closest ones to the mirror 101.
In FIG. 1C, which shows a second abnormal mode whose natural frequency is fn2 (>fn1) as shown in FIG. 2, the Z-axis position at a folded portion Fa23 between the piezoelectric cantilevers 102a-2 and 102a-3 is opposite to the Z-axis position at a folded portion Fb23 between the piezoelectric cantilevers 102b-2 and 102b-3 with respect to the origin O. Note that the folded portions Fa23 and Fb23 are the second closest ones to the mirror 101.
Similarly, in a third abnormal mode whose natural frequency is fn3 (>fn2) as shown in FIG. 2, the Z-axis position at a folded portion Fa12 between the piezoelectric cantilevers 102a-1 and 102a-2 is opposite to the Z-axis position at a folded portion Fb12 between the piezoelectric cantilevers 102b-1 and 102b-2 with respect to the origin O. Note that the folded portions Fa12 and Fb12 are the third closest ones to the mirror 101.
Additionally, when the Y-axis position at the folded portion Fa34 is opposite to the Y-axis position at the folded portion Fb34 with respect to the origin O, when the Y-axis position at the folded portion Fa23 is opposite to the Y-axis position at the folded portion Fb23 with respect to the origin O, or when the Y-axis position at the folded portion Fa12 is opposite to the Y-axis position at the folded portion Fb12 with respect to the origin O, other natural frequencies which are not shown in FIG. 2 may be determined.
In FIG. 2, fn0/2 is a half natural frequency of the natural frequency fn0.
Thus, when saw-tooth wave control voltages having a frequency close to the natural frequency fn0 of the normal mode are applied to the piezoelectric actuators 102a and 102b, the saw-tooth wave control voltages would resonate with the natural frequency fn0 in the normal mode to enhance the deflection angle of the mirror 101. In this case, however, higher harmonic components of the saw-tooth wave control voltages would also resonate with the natural frequencies fn1, fn2, fn3, . . . in the abnormal modes so that such natural frequencies fn1, fn2, fn3, . . . would appear in the deflection angle of the mirror 101 as spurious noises.
In order to suppress the deformation of the meander-type piezoelectric actuators by an external impact and move away from the components of the natural frequencies fn1, fn2, fn3, . . . in the abnormal modes, a second prior art optical deflector as a MEMS device additionally includes damper elements thinly coated on the rear surfaces of the meander-type piezoelectric actuators. The damper elements are made of gel material with high viscosity. The gel material can be a heating or ultraviolet curable gel material which is cured by heating or ultraviolet irradiation (see: US2012/0120470A1 & JP2012-123364A).
In the above-described second prior art optical deflector, however, since a process for coating gel material thinly on the meander-type piezoelectric actuators and a process for curing the gel material are required, the manufacturing cost would be increased. Also, since the viscosity of gel material of one of the meander-type piezoelectric actuators would be different from that of the other, due to the manufacturing fluctuations, the cantilevers of one of the meander-type piezoelectric actuators would be asymmetrical to those of the other of the meander-type piezoelectric actuators, which would invite the interference of the piezoelectric actuators with the mirror and would twist or deform the meander-type piezoelectric actuators to be broken down. Further, since the damper elements cannot completely avoid the asymmetrical Z-axis motions of the folded portions, the components of the natural frequencies in the abnormal modes near the natural frequency in the normal mode cannot completely be moved away.